A controller for a high-frequency agitation source includes a signal generator to generate a drive signal having a variable duty cycle. The drive signal is used to drive the high-frequency agitation source. The controller also includes a temperature detector to detect the temperature of the high-frequency agitation source. The controller is configured to vary the duty cycle of the drive signal in response to the temperature of the high-frequency agitation source. By varying the duty cycle of the drive signal, the average power supplied to the piezoelectric crystal can be varied while still maintaining a fixed amplitude of oscillation. This allows the temperature of a high-frequency agitator, for example, a piezoelectric crystal, to be controlled.